1. Field of the Invention
One subject of the present invention is a catalytic reactor comprising a catalytic cellular structure, in particular a catalytic ceramic or metallic foam, and elements optimizing the contact with the inner wall of the reactor.
2. Related Art
Foams made of ceramic or even of metal alloy are known to be used as catalyst support in chemical reactions, in particular heterogeneous catalysis reactions. These foams are particularly beneficial for highly exothermic or endothermic reactions (e.g. the exothermic Fischer-Tropsch reaction, the water-gas shift reaction, partial oxidation reaction, methanation reaction, etc.), and/or for catalytic reactors where high space velocities are sought (steam reforming of natural gas, naphtha, LPG, etc.).
The most widespread method used to create ceramic foams with open macroporosity consists of impregnating a polymer foam (usually a polyurethane or a polyester foam), cut to the desired geometry, with a suspension of ceramic particles in an aqueous or organic solvent. The excess suspension is removed from the polymer foam by repeated application of a compression or by centrifugal spinning, so as to leave only a fine layer of suspension on the strands of the polymer. After one or more impregnations of the polymer foam using this method, the foam is dried to remove the solvent while maintaining the mechanical integrity of the deposited layer of ceramic powder. The foam is then heated to a high temperature in two stages. The first stage known as the binder removal stage consists in degrading the polymer and any other organic compounds that might be present in the suspension, through a slow and controlled increase in temperature until the volatile organic compounds have been completely eliminated (typically 500-900° C.). The second stage known as sintering consists in consolidating the residual inorganic structure using a high-temperature heat treatment.
This method of manufacture thus makes it possible to obtain an inorganic foam which is the replica of the initial polymer foam, give or take the shrinkage caused by the sintering. The final porosity achievable through this method covers a range from 30% to 95% for a pore size ranging from 0.2 mm to 5 mm. The final pore size (or open macroporosity) is derived from the macrostructure of the initial organic “template” (polymer foam, generally polyurethane foam). Said macrostructure generally varies from 60 to 5 ppi (ppi stands for pores per inch, the pores measuring from 50 μm to 5 mm).
The foam may also be of a metallic nature with a chemical formulation that allows the architecture to have chemical stability under operating conditions (temperature, pressure, gas composition, etc.). In the context of an application to the steam reforming of natural gas, the metallic cellular architecture will consist of chemical formulations based on NiFeCrAl oxidized at the surface, this surface oxidation making it possible to create a micron-scale layer of alumina that protects the metallic alloy from any corrosion phenomena.
Cellular architectures that are ceramic and/or metallic covered with ceramic are good supports for catalysts in numerous respects:                they have a maximum surface area/volume (m2/m3) ratio, so as to increase the geometric area for exchange and therefore indirectly increase the catalytic efficiency,        they minimize pressure drops along the bed (between the inlet and the outlet of the catalytic reactor),        they have heat transfer of improved axial and/or radial efficiency. Axial means along the axis of the catalytic reactor, and radial means from the internal or external wall of the catalytic reactor toward the center of the catalytic bed,        they improve the thermomechanical and/or thermochemical stresses withstood by the bed,        they improve the fill density of a tube by comparison with a random filling brought about by conventional structures (spheres, pellets, cylinders, barrels, etc.),        control of the filling makes it possible to ensure homogeneity of the filling from one tube to another.        
Nevertheless, one problem that is faced is the low quality, during operation, of the “physical” contact between the cellular architectures and the inner wall of the reaction chambers. This is a fortiori true for reactors operating at high temperatures, where the expansion of the metal tube will be greater than that of the cellular architecture of ceramic nature in particular. In the case of architecture of metallic cellular nature, the expansion of the two elements (catalytic bed, reaction chamber containing it) may be harmonized more easily.
This poor contact between the cellular architectures and the inner wall of the reaction chambers may pose two difficulties:                the creation of preferential flows at the wall, the reactants then not being in contact with the catalyst,        a poor radial heat transfer.        